Virtual Leaks And Why They Matter

A virtual leak is a gas source that is trapped physically in a chamber. Therefore, the trapped gas pocket’s path that goes into the chamber proper becomes small and low conductance. Virtual leaks often occur due to lack of abiding by the correct vacuum practice or a manufacturing fault in the material. There are various types of virtual leaks. For instance, an internal weld crack. If there is air being released, the gas in the crack will leave the crack slowly because there will be a restriction of gas easily flowing into the chamber proper.

Sometimes it is impossible to detect a virtual leak using a regular leak sensor. You may not be able to detect a virtual leak when carrying out a leak test after the assembly sequence, and then it will show up when you are doing a test of the pump down of the system. You probably have a virtual gas leak if the system’s final pressure is not what was expected, or it is not giving you a pumpdown curve.

Types of Virtual Leaks

It would be best to avoid a virtual leak during the design phase. However, this is not possible because a virtual leak could occur because of a simple factor such as a gap between two surfaces that are in contact. Since it is hard to avoid them, you have to learn to live with them. However, this does not mean that you should allow multiple leaks in your system. While working on the design ensure that you are doing away with any possible virtual leaks. Generally, gas leaks occur in the following ways.

• Trapped pockets
• Surface contacts
• Cracks
• Gaps

Leak Detection

There is no standard method that can be used to detect all forms of virtual leaks. There are different methods that are often employed to establish if there are any virtual leaks. These methods can test for leaks from low-vacuum pressure to high-vacuum pressure.

Bubble Test

Bubble tests are appropriate to use when testing for low-pressure virtual leaks. The best example of how a bubble test is carried out is getting a bicycle tube that is punctured, and placing it under water. The bubbles will form where the puncture is and you can mark that part.

Pump Down Test

The pump down test is carried out by clearing a vacuum vessel that is closed until a specific pressured is achieved, then closing the inlet valve of the pump. After a specific period elapses, the inlet valve will be opened, and the pump will return the vacuum to its original cleared level. The time it takes has to be recorded. The process will be repeated severally, and if the time stays constant then there is a virtual leak.

Helium Test

A helium leak sensor is the most reliable way to test for the smallest leaks. There are two ways in which one can use a helium leak sensor. These are integral testing and local testing. Integral testing is employed when the sample is under vacuum or pressure, and is contained in a vessel. Local testing is used when the sample is under vacuum or pressure.

The Importance of Eliminating Virtual Leaks

Using good assembling techniques for brazing and welding could potentially help one avoid virtual leaks. Different vacuum applications and processes require different leak rates. This means that while a certain leak rate may be okay at a lower vacuum it could possibly be dangerous and unacceptable at a higher vacuum level. Therefore, it is essential to use a leak sensor if you suspect that there is a virtual leak. Listed are reasons why it is important to eliminate virtual leaks.

• Eliminating leaks ensures that the process is efficient
• It ensures operator safety in case there are toxic fluids or gases
• Ensures quality and environmental standards are upheld
• It ensures there is a long products user life
• Ensures there is product safety for instance, air that could potentially contribute to an explosive mixture forming is prevented from entering the system

As mentioned earlier, it is vital to note that it is impossible for a system to be vacuum-tight. All you need to do is ensure that the virtual leaks are low and are manageable. Doing this ensures that the operating pressure, and the ability and balance of the gas to maintain and get to the final or ultimate pressure are not hugely impacted. If you notice there is a virtual leak and you can fix it, do that. While it is acceptable to live with a certain number of leaks, that does not mean you need to allow over ten leaks in your system.

You should also be able to discern the kind of leak on your system. Virtual leaks can either happen from the outside or the inside. In an outside leak, the fluid or gas flow’s direction is toward the vessel, this is also referred to an outside-in leak. An inside leak is also referred to as the inside-out leak, which is when the fluid or gas passes from the inside of the system going outwards. You could use either of two leak technologies, leak measurement and leak detection. If you do not have a leak detector, High Vac Depot offers an array of leak detector rentals.

Existing Virtual Leaks

When your system is operational and you detect a virtual leak, you have two options either start over or find an appropriate way to relieve the gas paths. The later is a more reasonable option. It is impossible to start over since it is economically unthinkable. Therefore, you would have to relieve the gas paths.

You can do this by rebuilding or modifying components that are demountable, or drilling holes on the chamber to relive the gas. Alternatively, you could release the chamber to argon if a virtual leak cannot be relieved. However, you need to employ this method if you are certain that argon will not be harmful to the process. Generally, you will follow a couple of the same techniques that were employed during the design phase.

It is vital to ensure that you are managing any virtual leaks that could be on a chamber. As aforementioned, it is not always easy to detect where a virtual leak is sometimes. So ensure that you use the appropriate equipment. High Vac Depot offers a range of leak detectors that you could either buy or rent.